There has been known a technique in which when an electric current passing through a discharge electrode is equal to or more than a predetermined value, it is determined that particulate matter (hereinafter also referred to as PM) has adhered to the discharge electrode, and an applied voltage to the electrode is caused to increase so as to remove the particulate matter from the discharge electrode (see, for example, a first patent document).
In addition, there has also been known another technique in which a discharge electrode is arranged in an exhaust passage of an internal combustion engine, and a corona discharge is caused to occur from the discharge electrode, whereby particulate matter is charged and condensed or aggregated (see, for example, a second patent document). By the condensation or aggregation of the particulate matter, the number of particles in the particulate matter can be decreased. Moreover, the diameters or sizes of particles in the particulate matter become large, so when a filter is arranged at a downstream side, it becomes easy to trap the particulate matter with the filter.
Here, there is a correlation between the electric current passing through the electrode, and the amount of particulate matter which has condensed or aggregated (i.e., this may also be the amount of decrease in the number of particles in the particulate matter). That is, the more the amount of aggregation of the particulate matter, the larger becomes the electric current passing through the electrode becomes. For this reason, the amount of aggregation of the particulate matter can be estimated from the electric current passing through the electrode. However, electricity flows through those substances other than the particulate matter which are contained in an exhaust gas. Accordingly, when the amount of aggregation of the particulate matter is estimated based on the electric current passing through the electrode, there will be a possibility that the accuracy of estimation may be made low due to the substances, other than the particulate matter, which are contained in the exhaust gas.